1. Field of the Invention
The present invention relates generally to a developing device for supplying a developer to a developer carrying member such as a developing sleeve by conveying the developer with a supplying screw in the rotational axial direction of the screw, and an image forming apparatus including the same.
2. Description of the Related Art
Conventionally, there is known a developing device provided with a developer carrying body including a rotatable tubular developing sleeve as the developer carrying member and a magnet roller that is provided inside the developing sleeve in such a manner as to not rotate along with the developing sleeve, and a supplying screw disposed near the developing sleeve. In such a configuration, the developing sleeve carries the developer on its surface with the magnetic force emitted by the magnet roller provided inside the developing sleeve. As the developing sleeve rotates, the developer is moved along the surface, and is conveyed to the developing position facing a latent image carrier such as a photoconductor, to be used for the developing operation. The supplying screw is disposed in such a manner that it faces the developing sleeve and its rotational axial direction is in line with the rotational axial direction of the developing sleeve. Along with the rotation of the supplying screw, the developer is conveyed along the axial direction of the supplying screw so as to be supplied to the developing sleeve. The supplying screw also collects, from the developing sleeve, the developer that is finished being used for the developing operation. The developer that has been conveyed to the edge of the supplying screw on the downstream side in the developer conveyance direction is passed on to another screw. After being replenished with toner, the developer is returned to the edge of the supplying screw on the upstream side in the developer conveyance direction. The developer is conveyed and circulated this manner. Accordingly, when the toner density of the developer decreases after the developer has been used for the developing operation on the developing sleeve, the developer is collected from the developing sleeve. Subsequently, the developer is replenished with toner so that the toner density is restored. Then, the developer is supplied to the developing sleeve once again.
Patent documents 1 and 2 describe a developing device in which the developer is collected from the developing sleeve and supplied to the developing sleeve with the use of different screws. In this developing device, a receiving screw disposed in such a manner as to face the developing sleeve is used to collect the developer that has been used for the developing operation on the developing sleeve before it is conveyed to a position facing the supplying screw. Then, the developer is returned to the supplying screw directly from the receiving screw or via another conveying screw.
In order to stabilize the amount of toner being conveyed to the developing position, a regulating member is typically provided, both in a configuration for using only the supplying screw to supply/collect the developer to/from the developing sleeve and in a configuration for using separate screws to supply/collect the developer to/from the developing sleeve. This regulating member is disposed, with a predetermined gap between the surface of the developing sleeve, in such a manner as to face a portion on the surface of the developing sleeve where the developer has passed the position facing the supplying screw and before the developer reaches the developing position. The developer on the developing sleeve passes through this gap while being conveyed by the rotation of the developing sleeve. Accordingly, the layer of the developer can be regulated to a predetermined thickness, so as to stabilize the amount of toner conveyed to the developing position.
In this configuration, the developer that has been hampered, by the regulating member, from being conveyed by the rotation of the developing sleeve, accumulates in the region extending from the position facing the supplying screw to the position facing the regulating member. The accumulated developer (hereinafter, “regulated/accumulated developer”) is scraped against developer that is subsequently supplied to the aforementioned region as the developing sleeve rotates, and therefore the regulated/accumulated developer receives pressure and a shearing force. If this continues for a long period of time, the external additive particles such as silica added to the toner particles in the developer will be gradually buried in the toner particles. Accordingly, a spent phenomenon will occur, in which the toner particles adhere to the magnetic carriers. Furthermore, the toner particles that have not adhered to the magnetic carriers will be scraped against the magnetic carriers and/or collide with the magnetic carriers. As a result, each of these toner particles will become abraded and round, thereby having degraded properties. When a spent phenomenon occurs or when the toner particles become abraded, the image quality becomes degraded. For example, images with missing portions may be formed.
In the developing devices described in patent documents 1 and 2, such degradation in image quality is mitigated as follows. Among the plural magnetic poles of the magnet roller, a regulating magnetic pole and the magnetic pole that is adjacent to the regulating magnetic pole on the upstream side in the developing sleeve rotational direction have the same polarity. Furthermore, the regulating magnetic pole also serves as a pump-up magnetic pole. Specifically, the regulating magnetic pole of the magnet roller faces the regulating member via the developing sleeve, and the regulating magnetic pole has a function of attracting the developer onto the surface of the developing sleeve at the position facing the regulating member. Moreover, the magnetic pole (hereinafter, “regulation upstream magnetic pole”) that is adjacent to the regulating magnetic pole on the upstream side in the developing sleeve rotational direction typically functions as a pump-up magnetic pole for attracting and pumping up the developer around the supplying screw to the surface of the developing sleeve. However, the developing devices described in patent documents 1 and 2 are not provided with a magnetic pole dedicated to the pumping-up function. They only include the regulating magnetic pole that also serves as a pump-up magnetic pole. Specifically, a regulating magnetic pole and a regulation upstream magnetic pole are made to have the same polarity, thereby forming a repulsion magnetic field in which these magnetic poles are not connected by a magnetic force line. The edge of the regulating magnetic pole on the upstream side in the developing sleeve rotational direction faces the supplying screw. The magnetic force line extending from this upstream side edge is caused to largely curve as it is repulsed from the adjacent regulation upstream magnetic pole. Subsequently, this magnetic force line passes over the regulating magnetic pole and curves into a regulation downstream magnetic pole on the opposite side. The magnetic force is relatively low at the edge part of the regulating magnetic pole on the upstream side, where the magnetic force line is caused to curve in the above described manner. Therefore, a spent phenomenon and toner abrasion in the regulated/accumulated developer can be mitigated.
Patent Document 1: Japanese Laid-Open Patent Application No. H11-194617
Patent Document 2: Japanese Laid-Open Patent Application No. 2003-287950
However, in such a configuration, even when the volume density of the developer is relatively low, the magnetic force at the position facing the regulating member is not strong enough to increase the volume of the developer. Therefore, the developer having low volume density is regulated in its layer thickness. For this reason, when the volume density of the developer changes due to environmental changes, the amount of toner conveyed to the developing position changes accordingly. As a result, it has been difficult to attain stable developing density.
Furthermore, if the supplying screw faces the above repulsion magnetic field, the developer may not be pumped up to the developing sleeve surface. For this reason, there are limitations in terms of the layout. That is, the supplying screw cannot be disposed to face a region extending between the regulating magnetic pole and the regulation upstream magnetic pole.